Parents want to train their children to go to toilet independently. Such act can increase the self-confidence of the children and it is also the entry requirement for nursery schools. However, most of the current toilet training systems are passive and inefficient. For instance, one of such systems trains children to stay for a certain time on their own potties while they can watch TV and read books on the potties simultaneously. Unfortunately, children usually do not want to go to toilet because they do not have enough urine or they have already discharged urine before the training and so toilet training fails. Also, children usually fails to show clear signal of urination to the parents until the very last moment and so parents cannot run the training as expected effectively; thus they may need an indicator for indicating the fullness of the children's bladders.
Devices for measuring the volume of urine in bladders are available. For example, U.S. Pat. No. 5,058,591 and U.S. Pat. No. 4,926,871 disclose techniques for the measurement of the volume of a human bladder utilizing ultrasound transducer. However, the implementation of ultrasonic transducer in such device may make it expensive and too bulky to be readily installed on children or ones in need thereof, and may also interrupt their movement.